Integrate qucsator_rf as Git submodule

[ra3xdh]

Qucsator still contain a lot of unique features for RF simulation and it is not possible to fully deprecate it. I have recently created a fork https://github.com/ra3xdh/qucsator_rf The development roadmap could be found here: ra3xdh/qucsator_rf#1 One of the points is to add the qucsator-rf as the Git submodule in the main tree. This is planned for the next release after 24.1.0

[ra3xdh]

Done via #646